I. Field
The following description relates generally to wireless communications, and more particularly to segmenting and/or concatenating radio link control (RLC) service data units (SDUs) into RLC protocol data units (PDUs), wherein the RLC SDUs typically have sizes that exceed 2047 bytes.
II. Background
Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data can be provided via such wireless communication systems. A typical wireless communication system, or network, can provide multiple users access to one or more shared resources (e.g., bandwidth, transmit power, . . . ). For instance, a system can use a variety of multiple access techniques such as Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), Orthogonal Frequency Division Multiplexing (OFDM), 3GPP Long Term Evolution (LTE) systems, and others.
Generally, wireless multiple-access communication systems can simultaneously support communication for multiple access terminals. Each access terminal can communicate with one or more base stations via transmissions on forward and reverse links The forward link (or downlink) refers to the communication link from base stations to access terminals, and the reverse link (or uplink) refers to the communication link from access terminals to base stations. This communication link can be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
MIMO systems commonly employ multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas can be decomposed into NS independent channels, which can be referred to as spatial channels, where NS≦{NT,NR}. Each of the NS independent channels corresponds to a dimension. Moreover, MIMO systems can provide improved performance (e.g., increased spectral efficiency, higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
MIMO systems can support various duplexing techniques to divide forward and reverse link communications over a common physical medium. For instance, frequency division duplex (FDD) systems can utilize disparate frequency regions for forward and reverse link communications. Further, in time division duplex (TDD) systems, forward and reverse link communications can employ a common frequency region so that the reciprocity principle allows estimation of the forward link channel from reverse link channel.
The Long Term Evolution (LTE) Layer 2 user-plane protocol stack is generally composed of three sub layers: the packet data convergence protocol (PDCP) layer; the radio link control (RLC) layer; and the medium access control (MAC) layer. Typically, the PDCP layer (currently the top of the Layer 2 protocol stack) processes radio resource control (RRC) messages in the control plane and Internet Protocol (IP) packets in the user plane. Depending on the radio bearer, the main functions of the PDCP layer are header compression, security, and support for reordering and retransmission during handover. The RLC layer generally provides segmentation and/or reassembly of upper layer packets in order to adapt them to a size which can actually be transmitted over the radio interface. For radio bearers which require error-free transmission, the RLC layer can also perform retransmission to recover from packet losses. Additionally, the RLC layer performs reordering to compensate for out-of-order reception due to hybrid automatic repeat request (HARD) operation in the MAC layer. The MAC layer (currently the bottom of the Layer 2 protocol stack) performs multiplexing of data from different radio bearers. By deciding the amount of data that can be transmitted from each radio bearer and instructing the RLC layer as to the size of packets to provide, the MAC layer aims to achieve the negotiated quality of service (QoS) for each radio bearer. For the uplink, this process can include reporting to the base station or eNodeB the amount of buffered data for transmission.
At the transmitting side, each layer can receive a service data unit (SDU) from a higher layer, for which the layer provides a service, and outputs a protocol data unit (PDU) to the layer below. For instance, the RLC layer can receive packets from the PDCP layer. These packets are typically called PDCP PDUs from a PDCP perspective and represent RLC SDUs from the RLC point of view. The RLC layer creates packets which are provided to the layer below (e.g., the MAC layer). The packets which the RLC provides the MAC layer are RLC PDUs from an RLC perspective, and MAC SDUs from the MAC point of view. At the receiving side the process is reversed, with each layer passing SDUs up the stack where they are received as PDUs.
An important design feature of the LTE protocol stack is that all the PDUs and SDUs are byte aligned (e.g., the lengths of the PDUs and SDUs are multiples of 8 bits). This is to facilitate handling by microprocessors, which are typically defined to handle packets in units of bytes. In order to further reduce the processing requirements of the user plane protocol stack in LTE, the headers created by each of the PDCP, RLC, and MAC layers are also byte aligned. This implies that sometimes unused padding bits are needed in the headers, and thus the cost of designing for efficient processing is that a small amount of potentially available capacity is wasted.